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Message started by Mekh on 01/13/17 at 08:45:14

Title: Piston ring end gap (free)
Post by Mekh on 01/13/17 at 08:45:14

Hi,

According to Clymers manual free end gap dimensions are:

Specification
Top 11.5mm
Second 14mm

Wear limit
Top 9.2mm
Second 11.2mm

Can someone please explain this to me? I do not understand why the wear limit distance is smaller than the specification distance.

I measure mine at about:
Top 11.5mm
Second 10.5mm

For information: gap distances inside the cylinder are just fine.

Title: Re: Piston ring end gap (free)
Post by youzguyz on 01/13/17 at 13:35:05

The rings have to expand against the cylinder wall to seal it off.
If the free gap is too small, it doesn't have enough expansion force to provide an effective seal.
In My Opinion, if you have the engine apart enough to measure the free gap, then get the cylinder checked by a machine shop.
Get it honed, and replace the rings.

Title: Re: Piston ring end gap (free)
Post by Mekh on 01/14/17 at 00:03:34

I will be measuring the cylinder and piston today, but I'd be surprised if it turns out to be worn significantly, based on the looks of them.

With regards to free end gap. What I cannot make sense of, is that the wear limit is a smaller distance than the spec. distance and not larger. Can someone explain this!?

Title: Re: Piston ring end gap (free)
Post by youzguyz on 01/14/17 at 01:17:46

2C040A09610 wrote:

Think of the ring as a compression type spring.
The larger the gap, the more force the spring can produce when it is compressed as it tries to return to it's "free" state.
As the ring gets older, it loses the ability to spring back to it's original size (the larger free end gap).
Therefore, smaller gap = more worn out.

Title: Re: Piston ring end gap (free)
Post by Dave on 01/14/17 at 03:34:43

Youzguys is correct, the "free end gap" is a measure of the "springiness" left in the ring. As the ring becomes fatigued is loses it's ability to open back up and apply pressure against the cylinder wall.

The free end gap however is not a measure of the wear on the piston ring, and that must be measured with the piston ring inside the top edge of the cylinder - but far enough down so you are not on the ridge at the top of the cylinder where the rings never travel. The ring end gap within the cylinder should be 0.30 - 0.45mm, and it is worn out when it reaches 1.0mm. The closer the rings are to 0.45mm the less worn they are....the closer they are to 1.0 mm the more worn out they are.

If you are re-using the rings.....you probably don't need to hone the cylinder as the rings are broken in and used to running in that cylinder. If you install new rings - you need to hone the cylinder so that the rings will have a surface that can allow them to break in properly.

Title: Re: Piston ring end gap (free)
Post by Mekh on 01/14/17 at 04:44:32

Thank you, that makes sense. I was not considering that it could loose its "springiness" over time. Must be some kind of temperature induced creep...

So... Considering the measurements in my OP, I suppose ring 1 is OK, but ring 2 is worn beyond the wear limit. Correct?

Title: Re: Piston ring end gap (free)
Post by Armen on 01/14/17 at 07:35:56

FWIW, the ring set is in the low $30's. If you are that far into the bike, it makes sense to swap in new ones.

Title: Re: Piston ring end gap (free)
Post by Mekh on 01/14/17 at 13:09:05

Around here it is 70-75$ for a ring set...

http://www.mc-importen.dk/wop/

Title: Re: Piston ring end gap (free)
Post by Armen on 01/14/17 at 14:21:21

Ouch!